Socketable bump grid array shaped-solder on copper spheres

ABSTRACT

An electronic component having a socketable bump grid array comprising shaped-solder coated metallic spheres is provided by a method which comprises positioning solder coated metal spheres in an aligning device having a plurality of openings corresponding to the array, the openings being tapered preferably in the form of a truncated cone with the base of the cone being at the upper surface of the aligning device and having a diameter larger than the diameter of the solder coated metal sphere. The opening is configured so that a sphere positioned in the opening extends partially above the upper surface of the aligning device. The pads of the substrate are then contacted with the positioned spheres and, when the solder is reflowed, the solder forms a bond between the conductive layer on the substrate in contact with the solder-coated metal sphere and takes the shape of the aligning device and which maintains a solder coating on the whole surface of the metal sphere. An apparatus is also provided for making such a socketable bump grid array.

This is a continuation divisional of copending application(s) Ser. No.08/674,559 filed on Jul. 2, 1996, now U.S. Pat. No. 5,868,304.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to electrically connecting oneelectronic component to another electronic component and, in particular,to providing a socketable bump grid array comprising shapedsolder-coated metal spheres attached to one of the components whichshaped solder-coated metal spheres are pluggable into sockets of theelectronic component to be connected thereto.

2. Description of Related Art

Forming an electronic package whereby an electrical component such as anintegrated circuit chip is electrically connected to a substrate such asa card, a board, another chip or any other electronic part is well-knownin the art. Surface mount technology (SMT) has gained acceptance as thepreferred means of joining electronic devices together, particularly inhigh end computers. As compared to more traditional pin connectionmethods, where a pin mounted to the backside of a ceramic module isthrust through a hole in the board, twice the number of modules can beplaced on the same board area using SMT.

A myriad of solder structures have been proposed for the surfacemounting of one electronic structure to another. Typical surface mountprocesses form the solder structures by screening solder paste onconductive, generally metallic pads exposed on the surface of a firstelectronic structure or substrate. A stencil printing operation is usedto align the contact mask to the pads. The solder paste areas on thescreened substrate are then aligned to corresponding pads on theelectronic structure or board to be connected. After alignment, thesubstrate and board go through a reflow operation to melt the solderpaste and create a solder bond between the corresponding pads on thesubstrate and board.

Other known surface mount technology uses solder balls rather than asolder paste to provide the solder connecting structures. By usingsolder balls, a more exact and somewhat greater quantity of solder canbe applied than through screening. The solder balls are aligned and areheld to a substrate and melted to form a solder joint on a conductivepad of the substrate. As before, the substrate with the newly joinedsolder balls is aligned to the board to be connected therewith and thesolder balls are then reflowed to form a solder bond between substrateand board. The use of solder ball connectors has been applied to themounting of integrated circuit chips using the so-called C-4 (controlcollapse chip connection) technology since the method and structure werefirst described and patented in U.S. Pat. Nos. 3,401,126 and 3,429,042of Miller, which patents are assigned to the present assignee.

Solder ball connector (SBC) technology for multilayer ceramic componentsis designed for low cost, low induction surface mounting interconnectionbut is not currently pluggable into a second level socket. SBC isspecifically designed for surface mount attach procedures since socketstypically require hard mating surfaces (like those found on pins) ratherthan the soft solder found on SBC's.

Socketing of SBC's is desirable on two accounts, however, the first isto provide module level burn-in of the semiconductor devices. The secondis to provide a field upgradeable package similar to the Pin Grid Array(PGA). SBC-like packaging is being considered for X-86 and Power PCtypes of microprocessors and lack of pluggability remains a major hurdleto use of this technology.

In IBM Technical Disclosure Bulletin, AT 885-0235 (September 1986) theuse of a copper ball surrounded by eutectic solder is shown as the jointstructure for attaching a multilayer ceramic (MLC) substrate to a PClaminate wherein the ball serves as a standoff. A similar concept isdescribed by Totta and Sopher for SLT technology as described in "SLTDevice Metallurgy and Its Monolithic Extensions" IBM .JRD, Vol. 13, No.3, pages 226-238, May 1969. Both techniques employ soldering together oftwo distinct components. Japanese patent no. 7,099,385 describes amanufacturing process of preventing crushing of an entire solder balldue to melting of solder and provides a simple connection structure inthe gap between connection terminals by using a metallic sphereprecoated with solder. The basic SBC structure and processes aredescribed in U.S. Pat. Nos. 5,060,844 and 5,118,027, which patents arehereby incorporated by reference.

Bearing in mind the problems and deficiencies of the prior art, it is anobject of the present invention to use solder ball connector technologyto make electronic components having shaped socketable solder bump gridarrays for electrically connecting the electronic component to anotherelectronic component.

It is a further object of the invention to provide an apparatus tofabricate a socketable bump grid array on an electronic component.

It is another object of the invention to provide a method for making asocketable solder bump grid array.

Other objects and advantages of the present invention will be readilyapparent from the following description.

SUMMARY OF THE INVENTION

The above and other objects, which will be apparent to those skilled inthe art, are achieved in the present invention which is directed in thefirst aspect to a method for fabricating a socketable bump grid array ona plurality of conductive pads on a surface of a substrate comprisingthe steps of:

providing a plurality of solder-coated metal spheres, the solder havinga lower melting point than the metal sphere;

positioning the spheres into a plurality of aligning cavities in analigning device, the aligning device having an upper surface and a lowersurface and the cavities being arranged in a pattern corresponding to apattern of the conductive pads on the substrate, the aligning cavitieshaving tapered sides with size of the cavity being larger at the uppersurface of the aligning device and wherein the taper and the oppositeend of the cavity is sized so that the solder-coated metal spheres whenpositioned in the aligning cavities extend partially above the uppersurface of the aligning device;

placing a substrate having conductive pads thereon in rough alignment tothe positioned metal spheres so that at least a part of eachsolder-coated metal sphere contacts a corresponding conductive pad onthe substrate;

maintaining the contact between the conductive pad and the solder-coatedmetal spheres with a minimum of force;

reflowing the solder to allow the surface tension generated by thesolder to align the conductive pads on the substrate and solder-coatedmetal spheres;

the reflowed solder-coated metal spheres forming a tapered socketablebump whereby the base of each bump is connected to the correspondingconductive pad of the substrate by solder, and the bumps have soldercoated walls having a shape corresponding to the shape of the taper ofthe aligning cavity and the top of each of the bumps have a coating ofsolder which top shape of the bump substantially conforms to the shapeof the metal-sphere.

In another aspect of the invention, an apparatus is provided forfabricating a socketable bump grid-array on a plurality of conductivepads on a surface of a substrate comprising:

an aligning device having upper and lower surfaces and openings thereinpreferably extending through the device for positioning and securing inplace a plurality of solder-coated metal spheres in a patterncorresponding to a pattern of the conductive pads on said substrate;

the openings in the aligning device having tapered sides wherein theopening at the upper surface of the device has a diameter larger thanthe diameter of the solder-coated metal spheres and the bottom of thetaper has a diameter less than the diameter of the soldered-coated metalspheres the opening being of a size so that when the solder-coated metalsphere is positioned in the opening the sphere extends partially abovethe upper surface of the aligning device;

aligning means for positioning the substrate in rough alignment to saidsolder-coated metal spheres so that at least part of each metal spherewill wet a corresponding conductive pad on the substrate;

optional pressure means to urge said substrate against saidsolder-coated metal spheres with a minimum force;

whereby when the positioned solder-coated metal spheres are reflowed,the surface tension generated by said molten solder forms a socketablebump so that the base of each bump is connected to the conductive pad bysolder, the walls of each bump correspond to the shape of the taper andthe top of each bump has a coating of solder which coating forms a shapewhich substantially conforms to the shape of the metal sphere.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention believed to be novel and the elementscharacteristic of the invention are set forth with particularity in theappended claims. The figures are for illustration purposes only and arenot drawn to scale. The invention itself, however, both as toorganization and method of operation, may best be understood byreference to the detailed description which follows taken in conjunctionwith the accompanying drawings in which:

FIG. 1 shows a socketable bump shaped-solder coated metal spherestructure on a metallization pad of a multilayer ceramic substrate.

FIGS. 2A-2C show a process of the present invention for forming asocketable solder-coated metal sphere joint on the conductive pad of asubstrate using an aligning device of the invention.

DESCRIPTION OF THE PERFERRED EMBODIMENT(S)

In describing the preferred embodiment of the present invention,reference will be made herein to FIGS. 1-2c of the drawings in whichlike numerals refer to like features of the invention. Features of theinvention are not necessarily shown to scale in the drawings.

The present invention can be used for providing solder joints onsubstrates for connecting the substrate to another component utilizingSBC technology. In the packaging area, there are plethora of electronicstructures which require connection to other similar electronicstructures or to other levels of packaging. For example, integratedcircuit chips might be mounted to a metallized ceramic substrate; a cardon which several integrated chips had been surface mounted is thensubsequently surface mounted to a board which provide interconnectioncircuitry to a main frame computer. For the sake of clarity andconsistency, the solder ball joint of the present invention will bedescribed as produced on a multilayer ceramic substrate which would thenbe pluggable into other electronic components such as circuit board.

The solder-coated metal spheres used in the invention comprise ametallic sphere having a coating of solder. For the process of thepresent invention, it is only required that the solder have a lowermelting point than that of the metallic sphere and that the metallicsphere doesn't melt when the solder is reflowed. Any metallic spherethat can be wet by solder such as copper and nickel may be used but itis preferred to use spherical copper balls. An example of a preferredsolder is eutectic solder of 37/63 weight percent lead/tin. However,there are a wide range of solder materials which would be suitable forthe present invention which are well known to the solder connectionpart.

In general, the solder-coated metallic spheres will range in diameterfrom about 0.020 to 0.050 inches but 0.041 inches is preferred.Typically, the metallic sphere will be of a diameter of about 0.020 to0.035 inches and have a solder coating of about 0.0005 to 0.005 inches.A preferred solder coated metallic sphere comprises a copper metalsphere about 0.035 inch in diameter and a 0.003 inch solder coatingresulting in solder coated metallic sphere of about 0.041 inch diameter.The metallic spheres will preferably have a uniform-size with atolerance of about ±0.001 inch. The making of copper spheres isdescribed in the Totta and Sopher article, supra, and in general thecopper spheres are made by melting somewhat irregular copper spheroidsand the surface tension reshapes each mass into a nearly perfect sphere.The copper particles are usually melted with an aluminum oxide powderwhich help to keep most of the spheres separated during the meltingprocess. The alumina is easily separated from the copper by screeningand is recycled. The copper spheres are screened through powder metalsieves and separated into appropriate size fractions depending on thedesired use. The desired range of sphere sizes is preferably fed into amicrometer separator which consists of two rotating, gradually divergingrollers and the spheres are dropped through the rollers into bins whichhave a grading capability of 0.0001 inch.

Likewise, solder-coated copper or other metallic spheres are produced byapplying a coating of solder onto the metal sphere and grading thesespheres based on the above tolerances. While several solder depositionprocesses are possible, a preferred method comprises using barrelplating wherein electric current is applied to the metallic spheres byintermittent contact to a rotating barrel-all of which is submerged inan appropriate electroplating bath.

The aligning device used to position the spheres during manufacture ofthe bump grid array substrate is a generally planar flat device havingan upper surface and a lower surface with holes in the devicecorresponding to the desired bump grid array. The device may be madefrom any suitable material and graphite is a preferred material since ithas a good coefficient of thermal conductivity and its coefficient ofexpansion is compatible with ceramic substrates. The high thermalconductivity of graphite speeds the reflow process by conducting thefurnace heat to the solder joint. It also helps to maintain a consistentreflow temperature across the substrate which is important to maintainjoint composition uniformity. Some ceramic materials would be compatiblefor the aligning device, however, they are very expensive tomanufacture. Also, ceramic materials tend to be fragile while graphiteis substantially stronger and resists breakage.

The aligning device may be of any suitable thickness and is generallyabout 0.100 to 0.500 inch thick. As will be more fully discussedhereinbelow, the aligning device has a plurality of openings or cavitiesinto which the soldercoated metal spheres are positioned for connectionto pads on a substrate to form the bump grid array. The cavities havetapered sides basically in the shape of a cone with the larger diameterof the cavity being at the upper surface of the device. The bottom ofthe cavity has a diameter smaller than the diameter of the sphere andthe size of the cavity is such that when the solder-coated metal sphereis positioned therein that the top of the sphere partially extends abovethe upper surface of the alignment device. At the bottom of the cavityis an opening through which a vacuum may be applied to secure the spherein the cavity. The diameter of the vacuum through hole is not importantso long as sufficient vacuum force can be maintained on the metal sphereto hold it in place.

The alignment cavity is configured so that only a small portion of thesolder-coated metallic sphere extends beyond the upper surface of thealigning device on the order of about 0.004 to 0.010 inch preferably0.007 inch. The angle of the taper of the cavity may vary widely and isgenerally about 60 to 80 degrees, preferably 70 to 75 degrees. Loadingof the solder-coated metallic spheres into the aligning device cavitiesmay be done using any suitable technique such as vibratory means and isgenerally performed using a squeegee whereby an excess amount of spheresare placed on the surface of the aligning device and a squeegee movedacross the surface of the device thereby filling each cavity with asolder-coated metallic sphere. The squeegee is brought across in contactwith the upper surface of the aligning device and sweeps the excessmetallic spheres away.

Use of the aligning device of the invention provides a process forproducing a socketable bump grid array which is highly efficient andreliable. Fewer process defects are encountered using the presentinvention and the precision machining of the aligning device allows theball to ball centrality to be maintained with high accuracy. With theprocess of the present invention, virtually no bridging or missingsolder connections occur so long as the substrate is pressed against thesolder-coated metallic sphere with a minimum of force.

Referring now to FIG. 1, a socketable bump grid array assembly is showngenerally as 10. The assembly comprises a multilayer ceramic (MLC)substrate 11 having conductive pads 12 on one surface thereof. Asocketable bump shown generally as 13 comprises a solder layer 14 over ametal sphere 15. The bump 13 is shown as having a base 14a in the formof a fillet which is attached to and adheres to conductive pad 12. Thebump has walls 14b which are inwardly tapered from the base 14a endingin a curved shape top portion 14cwhich shape substantially conforms tothe shape of the metal sphere 15.

Referring to FIGS. 2a-2c, the process of the present invention isdescribed. FIG. 2a shows a solder-coated metal sphere generally as 16which comprises a metal sphere 15 covered by a substantially uniformsolder coating 14. In FIG. 2b the solder-coated metal sphere 16 ispositioned in alignment device 17 so that the metal sphere 16 extendspartially above the upper surface 18 of the alignment device. A vacuumwould be applied through opening 20 to hold the solder-coated metalsphere in place. A substrate 11 having a conductive pad 12 thereon isshown contacting base segment 14a of the solder coated metal sphere 16.Side walls 14b of sphere 16 are shown tangentially contacting thetapered walls 19 of alignment device 17. The top segment 14c of soldercoated metal sphere 16 is shown extending into the open recess 20 ofalignment device 17.

When the loaded solder coated metal sphere as shown in FIG. 2b isreflowed the solder melts (but not the metal sphere) and forms the bump13 having the tapered shape shown in FIG. 2c. As can be seen from thefigure, the lower portion of the bump 14a conforms to the shape of theconductive pad 12 and form a fillet. The bump walls 14b conform to theshape of taper walls 19 of alignment device 17. The top of the bump 14cis covered with solder and coats the metal sphere 15 which top 14c has ashape which substantially conforms to the shape of the metal sphere.

While the present invention has been particularly described, inconjunction with a specific preferred embodiment, it is evident thatmany alternatives, modifications and variations will be apparent tothose skilled in the art in light of the foregoing description. It istherefore contemplated that the appended claims will embrace any suchalternatives, modifications and variations as falling within the truescope and spirit of the present invention.

Thus, having described the invention, what is claimed is:
 1. Aelectronic component containing a socketable bump grid array on aplurality of conductive pads on a surface of the component made by amethod comprising the steps of:providing a plurality of solder-coatedmetal spheres, the solder having a lower melting point than the metalsphere; positioning the spheres into a plurality of aligning cavities inan aligning device, the aligning device having an upper surface and alower surface and the cavities being arranged in a pattern correspondingto a pattern of the conductive pads on the substrate, the aligningcavities having tapered sides with the size of the cavity being largerat the upper surface of the aligning device and wherein the taper andthe opposite end of the cavity is sized so that the solder-coated metalspheres when positioned in the aligning cavities extend partially abovethe upper surface of the aligning device; placing a substrate havingconductive pads thereon in rough alignment to the positioned metalspheres so that at least a part of each solder coated metal spherecontacts a corresponding conductive pad on the substrate; maintainingthe contact between the conductive pad and the solder-coated metalspheres with a minimum of force; reflowing the solder to allow thesurface tension generated by the solder to align the conductive pads onthe substrate and solder-coated metal spheres; the reflowedsolder-coated metal spheres forming a socketable bump wherein the baseof each bump is connected to the corresponding conductive pad of thesubstrate by solder, and the bumps have solder coated sides having ashape corresponding to the shape of the taper of the aligning cavity andthe top of each of the bumps have a coating of solder which top shape ofthe bump substantially conforms to the shape of the metal-sphere.
 2. Theelectronic component of claim 1 wherein the solder coated metallicspheres have a diameter of about 0.020 to 0.050 inches.
 3. Theelectronic component of claim 2 wherein the metallic sphere is copper.4. The electronic component of claim 3 wherein the aligning cavity hasthe shape of a truncated cone with there being an opening between thetop of the cone and the lower side of the aligning device forapplication of a vacuum for holding the spheres in place.
 5. Theelectronic component of claim 4 where the angle of taper is about 60 to80 degrees.